Hello,

I have seen the following routines in a class and don't understand how they
work. For one, why two new() routines?  Which is executed?  Why not just use
one routine?  What does the mybase do in this case?  Bit confusing!  :) Thanks

        '<remarks/>
        Public Sub New()
            MyBase.New()
            Me.Url =
System.Configuration.ConfigurationSettings.AppSettings("ReportServerURL") +
"/ReportService.asmx"
        End Sub

        '<remarks/>
        Public Sub New(ByVal ReportServerURL As String)
            MyBase.New()
            Me.Url = ReportServerURL + "/ReportService.asmx"
        End Sub

These constructors have 2 different parameters. Someone might want to create
an instance of this class and pass in the URL for the report server, or not.

If they don't pass it in, they get the report server from the configuration
file.

Otherwise, whatever they passed in is used.

In both cases, the base class's constructor is called, to make sure that the
entire instance gets instantiated properly. The base class constructor may
do some work without which the class isn't usable.

Show quoteHide quote

TheBee wrote:
Show quoteHide quoteMyBase.New in this case is not strictly necessary, since it is simply
calling the base classes default constructor.  The only time that
MyBase.New is required is if the Base class can't be constucted without
parameters - in other words, there is no constructor that doesn't take
arguments.  You may optionally use it if you want to pass arguments to
the base class constuctor...  Maybe a small example to make this clear:

Option Strict On
Option Explicit On

Imports System

Module Module1

   ' Base with only a default constructor
   Private Class Base
        Public Sub New(ByVal param As String)
            Console.WriteLine("Base New - {0}", param)
        End Sub
    End Class

   Private Class Descendant
        Inherits Base

        Public Sub New()
            Console.WriteLine("Descendant New")
        End Sub

        Public Sub New(ByVal param As String)
            Console.WriteLine("Descendant New - {0}", param)
        End Sub

    End Class

   Sub Main()
        Dim a As New Descendant
        Dim b As New Descendant("hi")
    End Sub

End Module

Now...  If you were to add a constructor to the Base class that took
parameters...

Imports System

Module Module1

    Private Class Base
        Public Sub New()
            Console.WriteLine("Base New")
        End Sub

        Public Sub New(ByVal param As String)
            Console.WriteLine("Base New - {0}", param)
        End Sub
    End Class

    Private Class Descendant
        Inherits Base

        Public Sub New()
            Console.WriteLine("Descendant New")
        End Sub

        Public Sub New(ByVal param As String)
            ' for fun, lets pass on param to the base!
            MyBase.New(param)
            Console.WriteLine("Descendant New - {0}", param)
        End Sub

    End Class
    Sub Main()
        Dim a As New Descendant
        Dim b As New Descendant("hi")
    End Sub

End Module

No, if you remove the Base.New () :

Option Strict On
Option Explicit On

Imports System

Module Module1

    Private Class Base

        Public Sub New(ByVal param As String)
            Console.WriteLine("Base New - {0}", param)
        End Sub
    End Class

    Private Class Descendant
        Inherits Base

        Public Sub New()
            MyBase.New("default")
            Console.WriteLine("Descendant New")
        End Sub

        Public Sub New(ByVal param As String)
            MyBase.New(param)
            Console.WriteLine("Descendant New - {0}", param)
        End Sub

    End Class
    Sub Main()
        Dim a As New Descendant
        Dim b As New Descendant("hi")
    End Sub

End Module

Anyway...  Only the last example REQUIRES the call to MyBase.New.  Most
times, this  is simply a convienient way to base arguments to a base
class constructor.  I think a lot of people still explicitly call
MyBase.New() simply because in some of the early VB.NET's (pre 1.0) it
was required...

Tom Shelton wrote:
Show quoteHide quoteCrap...  I copied in the wrong constructor..  That should simply be:

' Base with only a default constructor
Public Sub New()
    Console.WriteLine("Base New")
End Sub

Sorry for the confusion!

Thank you very much for the time you spent replying.  It is greatly
appreciated.

So the person who wrote the code really didn't need to do both.  They could
have just used the first one?  So you can duplicate a name of a routine many
times without issue?  Does compiler than automatically choose which to use
based on parameters passed?  What if both routines have same parameters with
types?  What happens then?

Thanks again

Show quoteHide quote

The Bee,
Yes this is known as overloading.

Yes the compiler looks at the number of parameters & their types to decide
which overload to call.

The signatures of the methods (routines) need to be unique on number & type
of parameters. Overloading on return value or parameter name will cause a
compile error.


Advanced information:

..NET 2.0 (VS 2005) allows you to overload generic classes on the number of
type parameters, however you cannot overload a generic simply on the
constraint. For example:

    System.Nullable - has shared methods pertaining to all nullable types.

    System.Nullable(Of T) - creates a specific nullable type, such as
Nullable(Of Integer) & Nullable(Of Double).

The reason for the two Nullable types is to simplify calling of the methods
on System.Nullable...

Show quoteHide quoteSystem.Configuration.ConfigurationSettings.AppSettings("ReportServerURL")
Show quoteHide quote

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